Magneto-optical recording medium with multiple magnetic layers

ABSTRACT

If the Curie points of the first magnetic layer, second magnetic layer, third magnetic layer and fourth magnetic layer of alloys of rare-earth metal and transition metal as ferrimagnetic materials showing perpendicular magnetization from room temperature to their Curie points are indicated as Tc1, Tc2, Tc3 and Tc4, respectively, the Curie points and room temperature are related by: room temperature&lt;Tc3&lt;Tc4&lt;Tc1&lt;Tc2. If the sublattice magnetization of transition metal is indicated as α and the sublattice magnetization of rare-earth metal is β, α is stronger than β in the second magnetic layer at temperatures between Tc1 and Tc2, and β is stronger than α in the fourth magnetic layer at temperatures between room temperature and Tc4. This structure enables light-intensity modulation overwriting, eliminates the necessity of orienting the magnetization of magnetic layers in one direction using a great magnetic field or high laser power before shipment from factories or before recording, and reduces an increase in the cost of manufacturing a magneto-optical recording medium and a device for recording information on the magneto-optical medium.

FIELD OF THE INVENTION

The present invention relates to a magneto-optical recording medium anda magneto-optical recording method used for optical disks, optical cardsand the like, for optically performing at least recording, reproducingor erasing of information.

BACKGROUND OF THE INVENTION

A magneto-optical recording system uses a recording medium formed bydepositing a perpendicularly magnetized film made of a magnetic materialon a substrate, and performs recording and reproducing operations in themanner mentioned below.

In order to perform recording, the recording medium is initialized by,for example, a strong external magnetic field, and the direction ofmagnetization is oriented in one direction (upward or downwarddirection). Thereafter, a desired recording area is irradiated with alaser beam to increase the temperature of the medium in the area to atleast near its Curie point or compensation point so that the coerciveforce (Hc) in the area becomes zero or substantially zero. Subsequently,an external magnetic field (bias magnetic field) opposite to theinitialized magnetization direction is applied to reverse themagnetization direction. When the irradiation of the laser beam isstopped, the recording medium returns to ordinary temperature, and thereversed magnetization is fixed. Thus, information is thermomagneticallyrecorded.

In order to perform reproduction, the recording medium is irradiatedwith a linearly polarized laser beam, and information is optically readusing such phenomena that the plane of polarization of light reflectedfrom or transmitted through the recording medium is rotated (Kerrmagnetic effect and Faraday magnetic effect).

The magneto-optical recording system has been focused as a rewritablelarge-capacity memory element. As a system for reusing (rewriting) therecording medium, a so-called light-intensity modulation overwritablemedium was proposed. The light-intensity modulation overwritable mediumenables overwriting by using an exchange-coupled-two-layer film, aninitialization magnetic field (Hi) and a recording magnetic field (Hw)and by performing light-intensity modulation. Further, a light-intensitymodulation overwritable medium of another type was also proposed. Thislight-intensity modulation overwritable medium includes anexchanged-coupled-four-layer film, and performs overwriting withoutusing an initializing magnetic field (Hi).

Referring now to FIGS. 16 to 18, the following description will brieflyexplain the process of light-intensity modulation overwriting using thelight-intensity modulation overwritable medium which includes theexchange-coupled four-layer film and requires no initializing magneticfield Hi.

As illustrated in FIG. 16, the light-intensity modulation overwritablemedium includes a first magnetic layer 13, a second magnetic layer 14, athird magnetic layer 15, and a fourth magnetic layer 16. The temperaturedependence of the coercive forces of these magnetic layers is shown inFIG. 17.

Next, the changes in the magnetization state of each magnetic layer willbe explained with reference to FIG. 18. The arrows in FIG. 18 show themagnetization direction of transition metal.

At room temperature, information is recorded depending on whether themagnetization direction of the first magnetic layer 13 is upward "0"(state S71) or downward "1" (state S77). The magnetization of the fourthmagnetic layer 16 is always oriented in one direction (an upwarddirection in FIG. 18), and the magnetization of the second magneticlayer 14 is oriented in the same direction as that of the fourthmagnetic layer 16 through the third magnetic layer 15.

Recording is performed by irradiating laser light whose intensity hasbeen modulated to high power or low power while applying the recordingmagnetic field Hw.

The high power and low power are set so that the medium is heated to atemperature near Curie point Tc2 of the second magnetic layer 14 (stateS74) when laser light of high power is irradiated, and heated to atemperature near Curie point Tc1 of the first magnetic layer 13 (stateS73) when laser light of low power is irradiated.

Therefore, when the laser light of high power is irradiated, themagnetization of the second magnetic layer 14 is switched to a downwarddirection by the recording magnetic field Hw (state S75), and copied tothe first magnetic layer 13 by an exchange force acting on the interfaceduring a cooling process (state S76). Then, the magnetization of thesecond magnetic layer 14 is oriented in the same direction as that ofthe fourth magnetic layer 16 (state S77). As a result, the firstmagnetic layer 13 shows the downward magnetization direction "1".

On the other hand, when the laser light of low power is irradiated, themagnetization of the second magnetic layer 14 is not switched by therecording magnetic field Hw because its coercive force is stronger thanthe recording magnetic field Hw (state S73). Similarly to the abovecase, the magnetization direction of the first magnetic layer 13 isaligned with the magnetization direction of the second magnetic layer 14by the exchange force acting on the interface during the cooling process(state S72). Therefore, the first magnetic layer 13 shows the upwardmagnetization direction "0" (state S71).

The laser power used for reproduction is set to a level much lower thanthe low power for recording.

Hence, the above-mentioned conventional technique uses anexchanged-coupled-four-layer film, and provides a magneto-opticalrecording medium capable of being overwritable by light-intensitymodulation without requiring the initializing magnetic field Hi and ofachieving stable recording bits.

In this conventional technique, however, it is necessary to orient themagnetization of the fourth magnetic layer 16 in one direction using alarge magnetic field or high laser power before shipping from factoriesor before recording. Consequently, the conventional technique suffersfrom such a drawback that the costs of manufacturing the magneto-opticalrecording medium and a device for recording information on the mediumincrease.

Moreover, if the direction of the magnetization of the fourth magneticlayer 16 which has been oriented in one direction is disordered for somereasons, light-intensity modulation overwriting cannot be carried out.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a magneto-opticalrecording medium capable of achieving light-intensity modulationoverwriting, eliminating the necessity of orienting the magnetization ofmagnetic layers in one direction using a great magnetic field(initializing magnetic field) or high laser power before shipment fromfactories or before recording and of reducing an increase in the cost ofmanufacturing the magneto-optical recording medium and a device forrecording information on the magneto-optical medium.

In order to achieve the above object, a first magneto-optical recordingmedium of the present invention includes a first magnetic layer, asecond magnetic layer and a fourth magnetic layer having Curie pointsTc1, Tc2 and Tc4, respectively, and showing perpendicular magnetizationfrom room temperature to Tc1, Tc2 and Tc4, the first, second and fourthmagnetic layers being arranged in this order, the direction ofmagnetization of the second magnetic layer being copied to the firstmagnetic layer by an exchange force at temperatures between roomtemperature and Tc1, the direction of magnetization of the fourthmagnetic layer being copied to the second magnetic layer by an exchangeforce but the magnetization of the second magnetic layer is not copiedto the first magnetic layer in a predetermined temperature range Rbetween room temperature and Tc4, room temperature, Tc1, Tc2 and Tc4being related by

    room temperature<Tc4<Tc1<Tc2.

A second magneto-optical recording medium of the present invention isbased on the first magneto-optical recording medium, and ischaracterized in that the second and fourth magnetic layers are made ofalloys of rare-earth metal and transition metal as ferrimagneticmaterials, if sublattice magnetization of one of the transition metaland the rare-earth metal is indicated as α and the other is β, α isstronger than β in the second magnetic layer at temperatures between Tc1and Tc2, and β is stronger than α in the fourth magnetic layer attemperatures between room temperature and Tc4.

A magneto-optical recording method of the present invention uses themagneto-optical recording medium of the present invention and ischaracterized in that a low process of irradiating a light beam of lowlevel for heating the magneto-optical recording medium to a temperaturenear Tc1 and a high process of irradiating a light beam of high levelfor heating the magneto-optical recording medium to at least atemperature near Tc2 are performed while applying a recording magneticfield Hw smaller than the coercive force of the second magnetic layer attemperature below Tc1.

In this structure and method, recording is performed according to thefollowing low process and high process for irradiating light beams oftwo levels of intensity, high and low levels. Here, in the upward anddownward directions perpendicular to the respective layers, if thedirection of magnetization of the second magnetic layer before heated isindicated as A and the opposite direction is B, the recording magneticfield Hw is applied in the B direction.

The low process is performed as follows.

The respective layers are heated to temperatures near Tc1 so as toextinguish the magnetization of each of the first and fourth magneticlayers. At this time, since the coercive force of the second magneticlayer is greater than the recording magnetic field Hw, the magnetizationof the second magnetic layer is not reversed. Moreover, since a isstronger than β in the second magnetic layer, α is oriented in the Adirection.

The respective layers are cooled down to temperatures between Tc4 andTc1 so as to copy the direction of each sublattice magnetization of thesecond magnetic layer to the first magnetic layer by the exchange force.As a result, in the first magnetic layer, α is oriented in the Adirection and β is oriented in the B direction.

The respective layers are cooled down to temperatures in theabove-mentioned temperature range R so as to orient the magnetization ofthe fourth magnetic layer in the B direction by the recording magneticfield Hw. At this time, since β is stronger than α in the fourthmagnetic layer, β is oriented in the B direction.

The high process is performed as follows.

The respective layers are heated to temperatures near Tc2 so as toextinguish the magnetization of all of the magnetic layers.

The respective layers are cooled down to temperatures between Tc1 andTc2. Since the magnetization of the second magnetic layer has beentemporarily extinguished, in contrast with the low process, thedirection of the second magnetic layer is oriented in the B direction bythe recording magnetic field Hw. At this time, since α is stronger thanβ in the second magnetic layer, α is oriented in the B direction.

The respective layers are cooled down to temperatures between Tc4 andTc1 so as copy each sublattice magnetization of the second magneticlayer to the first magnetic layer by the exchange force. As a result, inthe first magnetic layer, α is oriented in the B direction and β isoriented in the A direction.

The respective layers are cooled down to temperatures in theabove-mentioned temperature range R so as to orient the magnetization ofthe fourth magnetic layer in the B direction by the recording magneticfield Hw. At this time, since β is stronger than α in the fourthmagnetic layer, β is oriented in the B direction. Each sublatticemagnetization of the fourth magnetic layer is copied to the secondmagnetic layer by the exchange force. As a result, in the secondmagnetic layer, α is oriented in the A direction and β is oriented inthe B direction like the fourth magnetic layer. Thus, the direction ofthe magnetization of the second magnetic layer is initialized. At thistime, each sublattice magnetization of the second magnetic layer is notcopied to the first magnetic layer by the exchange force.

As described above, in the first magnetic layer, α is oriented in the Adirection and β is oriented in the B direction by the low process. Onthe other hand, when the high process is performed, α is oriented in theB direction and β is oriented in the A direction in the first magneticlayer. Namely, light-intensity overwriting can be performed.

As known from the above explanation, the magnetization of the fourthmagnetic layer can be oriented in a predetermined one direction, i.e.,initialized, by the recording magnetic field Hw before the direction ofthe magnetization of the second magnetic layer is initialized. Morespecifically, even if the magnetization of the fourth magnetic layer istemporarily extinguished with a rise in temperature, the initialmagnetization direction is restored before the initialization of thedirection of the magnetization of the second magnetic layer.

Therefore, the direction of the magnetization of the fourth magneticlayer can be controlled by a recording magnetic field that is a smallmagnetic field for recording without using a great magnetic field like aconventional initializing magnetic field and by a light beam powerweaker than a conventional power.

Accordingly, it is possible to achieve light-intensity modulationoverwriting, eliminate the necessity of orienting the magnetization ofmagnetic layers in one direction using a great magnetic field(initializing magnetic field) or high laser power before shipment fromfactories or before recording, and reduce an increase in the cost ofmanufacturing the magneto-optical recording medium and a device forrecording information on the magneto-optical medium.

Moreover, even when the direction of the magnetization of the fourthmagnetic layer is disordered for some reasons, it is possible to performlight-intensity modulation overwriting.

In addition to the above-mentioned first structure, the secondmagneto-optical recording medium of the present invention ischaracterized in that a third magnetic layer is provided between thesecond magnetic layer and the fourth magnetic layer, third magneticlayer showing perpendicular magnetization from room temperature to itsCurie point Tc3, Tc3 being related to room temperature and Tc4 by

    room temperature<Tc3<Tc4,

the temperature range R is a range between room temperature and Tc3, andthe magnetization of the fourth magnetic layer is copied to the thirdmagnetic layer by an exchange force and the magnetization of the thirdmagnetic layer is copied to the second magnetic layer by an exchangeforce in the temperature range R.

In this structure, like the first structure, recording is performedaccording to the following low process and high process of irradiatinglight beams of two levels of intensity, high and low levels. Here,similarly to the above explanation, in the upward and downwarddirections perpendicular to the respective layers, if the direction ofmagnetization of the second magnetic layer before heated is indicated asA and the opposite direction is B, the recording magnetic field Hw isapplied in the B direction.

The low process is performed as follows.

The respective layers are heated to temperatures near Tc1 so as toextinguish the magnetization of each of the first, third and fourthmagnetic layers. At this time, since the coercive force of the secondmagnetic layer is greater than the recording magnetic field Hw, themagnetization of the second magnetic layer is not reversed. Moreover,since α is stronger than β in the second magnetic layer, α is orientedin the A direction.

The respective layers are cooled down to temperatures between Tc4 andTc1 so as to copy the direction of each sublattice magnetization of thesecond magnetic layer to the first magnetic layer by the exchange force.As a result, in the first magnetic layer, α is oriented in the Adirection and β is oriented in the B direction.

The respective layers are cooled down to temperatures between Tc3 andTc4 so as to orient the magnetization of the fourth magnetic layer inthe B direction by the recording magnetic field Hw. At this time, sinceβ is stronger than α in the fourth magnetic layer, β is oriented in theB direction.

The respective layers are cooled down to temperatures below Tc3 so as tocopy each sublattice magnetization of the fourth magnetic layer to thethird magnetic layer by the exchange force.

The high process is performed as follows.

The respective layers are heated to temperatures near Tc2 so as toextinguish the magnetization of all of the magnetic layers.

The respective layers are cooled down to temperatures between Tc1 andTc2. Since the magnetization of the second magnetic layer has beentemporarily extinguished, in contrast with the low process, thedirection of the second magnetic layer is oriented in the B direction bythe recording magnetic field Hw. At this time, since α is stronger thanβ in the second magnetic layer, α is oriented in the B direction.

The respective layers are cooled down to temperatures between Tc4 andTc1 so as copy each sublattice magnetization of the second magneticlayer to the first magnetic layer. As a result, in the first magneticlayer, α is oriented in the B direction and β is oriented in the Adirection.

The respective layers are cooled down to temperatures between Tc3 andTc4 so as to orient the magnetization of the fourth magnetic layer inthe B direction by the recording magnetic field Hw. At this time, sinceβ is stronger than α in the fourth magnetic layer, β is oriented in theB direction.

The respective layers are cooled down to temperatures below Tc3 so as tocopy each sublattice magnetization of the fourth magnetic layer to thethird magnetic layer by the exchange force. Each sublatticemagnetization of the third magnetic layer is copied to the secondmagnetic layer by the exchange force. Therefore, in the second magneticlayer, α is oriented in the A direction and β is oriented in the Bdirection like the fourth magnetic layer. Thus, the direction of themagnetization of the second magnetic layer is initialized. At this time,each sublattice magnetization of the second magnetic layer is not copiedto the first magnetic layer by the exchange force.

As described above, like the above-mentioned first structure, in thefirst magnetic layer, α is oriented in the A direction and β is orientedin the B direction by the low process. On the other hand, when the highprocess is performed, α is oriented in the B direction and β is orientedin the A direction in the first magnetic layer. Namely, light-intensityoverwriting can be performed.

Moreover, since the third magnetic layer is provided between the secondmagnetic layer and the fourth magnetic layer, when the coercive force ofthe first magnetic layer is not sufficiently strong in the temperaturerange R in the course of lowering of the temperature of the magneticlayers, it is possible to prevent the magnetization of the fourthmagnetic layer from being copied to the first magnetic layer through thesecond magnetic layer by the exchange force from the fourth magneticlayer. Consequently, the first magnetic layer can be selected from awider range of materials.

In addition to the above-mentioned first or second structure, the thirdmagneto-optical recording medium of the present invention ischaracterized in that the sublattice magnetization of the rare-earthmetal is stronger than the sublattice magnetization of the transitionmetal at room temperature and the sublattice magnetization of thetransition metal is stronger than the sublattice magnetization of therare-earth metal at temperatures between Tc1 and Tc2 in the secondmagnetic layer, and the sublattice magnetization of the rare-earth metalis stronger than the sublattice magnetization of the transition metal attemperatures between room temperature and Tc4 in the fourth magneticlayer.

In this structure, when the temperature is lowered to room temperature,since the sublattice magnetization of the rare-earth metal of the fourthmagnetic layer is oriented in the B direction by the recording magneticfield Hw, the sublattice magnetization of the transition metal isoriented in the A direction. Therefore, the fourth magnetic layer in thestructure of claim 1, and the fourth and third magnetic layers in thestructure of claim 2 act for orienting the sublattice magnetization ofthe transition metal of the second magnetic layer in the A direction bythe exchange force.

On the other hand, the recording magnetic field Hw is exerted in adirection so that the magnetization of the second magnetic layer isoriented in the B direction. Therefore, the recording magnetic fieldacts for orienting the sublattice magnetization of the rare-earth metalof the second magnetic layer in the B direction. In other words, therecording magnetic field acts for orienting the sublattice magnetizationof the transition metal of the second magnetic layer in the A direction.

Thus, since the above-mentioned two actions are combined, therequirements for the recording magnetic field and the magnetic layersare eased. Namely, it is possible to decrease the strength of therecording magnetic field and the strength of the exchange forces of thefourth and third magnetic layers exerted on the second magnetic layer.As a result, the materials used for the magneto-optical disk can beselected from a wider range, and the increase in the cost ofmanufacturing a device for recording information on the medium can befurther reduced.

In addition to the above-mentioned first, second or third structure, thefourth magneto-optical recording medium of the present invention ischaracterized in that the fifth magnetic layer having Curie point Tc5higher than the Curie point Tc1 of the first magnetic layer is formed ona side of the first magnetic layer, opposite to a side whereupon thesecond magnetic layer is formed.

In this structure, since the Curie point of the fifth magnetic layer ishigher than the Curie point of the first magnetic layer, the Kerrrotation angle becomes greater when performing reproduction. As aresult, the signal quality is improved.

In addition to any of the above-mentioned structures 1 to 4, the fifthmagneto-optical recording medium of the present invention ischaracterized in that a sixth magnetic layer showing in-planemagnetization at room temperature and perpendicular magnetization at atemperature near Tc1 is provided between the first magnetic layer andthe second magnetic layer.

In this structure, since the sixth magnetic layer shows perpendicularmagnetization at a temperature near Tc1, the magnetization isefficiently copied from the second magnetic layer to the first magneticlayer. Additionally, since the sixth magnetic layer shows in-planemagnetization at room temperature, the magnetization of the firstmagnetic layer that recorded information is not affected by the exchangeforce from the second magnetic layer at room temperature. It istherefore possible to improve the recording sensitivity. As a result,high-quality light-intensity modulation overwriting can be performed.

For a fuller understanding of the nature and advantages of theinvention, reference should be made to the ensuing detailed descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view showing an example of the structure of amagneto-optical disk of the present invention.

FIG. 2 is a graph showing the temperature dependence of the coerciveforces of the first to fourth magnetic layers of the magneto-opticaldisk shown in FIG. 1.

FIG. 3 is an explanatory view showing the magnetic states of the firstto fourth magnetic layers to explain the recording process on themagneto-optical disk shown in FIG. 1.

FIG. 4 is an explanatory view showing another example of the structureof a magneto-optical disk of the present invention.

FIG. 5 is a graph showing the temperature dependence of the coerciveforce of the fifth magnetic layer of the magneto-optical disk shown inFIG. 4.

FIG. 6 is an explanatory view showing the magnetic states of the firstto fifth magnetic layers to explain the recording process on themagneto-optical disk shown in FIG. 4.

FIG. 7 is an explanatory view showing still another example of thestructure of a magneto-optical disk of the present invention.

FIG. 8 is a graph showing the temperature dependence of the coerciveforce of the fifth magnetic layer of the magneto-optical disk shown inFIG. 7.

FIG. 9 is an explanatory view showing the magnetic states of the firstto fifth magnetic layers to explain the recording process on themagneto-optical disk shown in FIG. 7.

FIG. 10 is an explanatory view showing yet another example of thestructure of a magneto-optical disk of the present invention.

FIG. 11 is a graph showing the temperature dependence of the coerciveforce of the sixth magnetic layer of the magneto-optical disk shown inFIG. 10.

FIG. 12 is an explanatory view showing the magnetic states of the firstto fourth and sixth magnetic layers to explain the recording process onthe magneto-optical disk shown in FIG. 10.

FIG. 13 is an explanatory view showing another example of the structureof a magneto-optical disk of the present invention.

FIG. 14 is a graph showing the temperature dependence of the coerciveforces of the fifth and sixth magnetic layers of the magneto-opticaldisk shown in FIG. 13.

FIG. 15 is an explanatory view showing the magnetic states of the firstto sixth magnetic layers to explain the recording process on themagneto-optical disk shown in FIG. 13.

FIG. 16 is an explanatory view showing an example of the structure of aconventional magneto-optical disk.

FIG. 17 is a graph showing the temperature dependence of the coerciveforces of the first to fourth magnetic layers of the conventionalmagneto-optical disk.

FIG. 18 is an explanatory view showing the magnetic states of the firstto fourth magnetic layers to explain the recording process on theconventional magneto-optical disk.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment 1!

The following description will discuss one embodiment of the presentinvention with reference to FIGS. 1 to 3.

As illustrated in FIG. 1, a magneto-optical disk (magneto-opticalrecording medium) of this embodiment includes a transparent dielectriclayer 2, a first magnetic layer 3, a second magnetic layer 4, a thirdmagnetic layer 5, a fourth magnetic layer 6, and a protective layer 7,formed in this order on a light transmitting substrate 1. Actually, anovercoat film (not shown) is formed outside of the protective layer 7.The first magnetic layer 3, second magnetic layer 4, third magneticlayer 5, and fourth magnetic layer 6 are made of alloys of rare-earthmetal and transition metal as ferrimagnetic materials in which themagnetization of rare-earth metal and that of transition metal areanti-parallel to each other.

As illustrated in FIG. 2, the first magnetic layer 3 has a lower Curiepoint, Tc1, and a higher coercive force, Hc1, at room temperaturecompared to the second magnetic layer 4, and shows perpendicularmagnetization from room temperature to Tc1. The composition of the firstmagnetic layer 3 is such that it is transition metal rich at roomtemperature.

The second magnetic layer 4 has a Curie point, Tc2, higher than theCurie point Tc1 of the first magnetic layer 3, and shows perpendicularmagnetization from room temperature to Tc2. The composition of thesecond magnetic layer 4 is such that it is rare-earth metal rich at roomtemperature and has a compensation point between room temperature andTc2, and that it is transition metal rich between the compensation pointand Tc2.

The third magnetic layer 5 has the lowest Curie point, Tc3, among thefirst to fourth magnetic layers, and shows perpendicular magnetizationfrom room temperature to Tc3. The composition of the third magneticlayer 5 is such that it is transition metal rich at room temperature.

The fourth magnetic layer 6 has a Curie point, Tc4, which is higher thanTc3 and lower than Tc1, and shows perpendicular magnetization betweenroom temperature and Tc4. The composition of the fourth magnetic layer 6is such that it is rare-earth metal rich at room temperature and doesnot have a compensation point between room temperature and Tc4.

Next, the recording process in this embodiment will be discussed withreference to FIG. 3. FIG. 3 shows the magnetization states of the firstmagnetic layer 2, second magnetic layer 4, third magnetic layer 5, andfourth magnetic layer 6. The horizontal axis in FIG. 3 indicatestemperature. Since each layer is formed by an alloy of rare-earth metaland transition metal, the total magnetization, sublattice magnetizationα of the transition metal, and sublattice magnetization β of therare-earth metal are present. The arrows shown in FIG. 3 indicate thedirection of the sublattice magnetization a of the transition metal ofeach layer.

When performing light-intensity modulation overwriting using such amagneto-optical disk, information is rewritten using an overwritingtechnique in which a high process and low process are repeatedlyperformed by modulating the intensity of a light beam according toinformation while applying the recording magnetic field Hw to a portionirradiated with the light beam. In the high process, the portionirradiated with the light beam is heated to a temperature near Tc2. Inthe low process, the portion irradiated with the light beam is heated toa temperature near Tc1.

At room temperature, two stable states, "0" (upward magnetization) and"1" (downward magnetization), are present depending on the direction ofthe sublattice magnetization of the first magnetic layer 3. These statesare S1 and S7 shown in FIG. 3.

In the high process, laser light of high power (Ph) is irradiated. As aresult, the temperature of the irradiated portion is increased to atemperature near Tc2, and the magnetizations of the first magnetic layer3, third magnetic layer 5, and fourth magnetic layer 6 become zero. Atthis time, the magnetization of the second magnetic layer 4 becomeszero, and is then oriented in a downward direction by the recordingmagnetic field Hw. Since the second magnetic layer 4 is transition metalrich around this temperature, the sublattice magnetization α of thetransition metal is oriented in a downward direction (S3, S4 and S5).

When the portion irradiated with the light beam is cooled down by arotation of the magneto-optical disk, the first magnetic layer 3 shows amagnetization. At this time, the sublattice magnetization of thetransition metal of the first magnetic layer 3 is oriented in a downwarddirection that is the direction of the sublattice magnetization of thetransition metal of the second magnetic layer 4 by an exchange forceexerted on the first magnetic layer 3 from the second magnetic layer 4at the interface between the first magnetic layer 3 and the secondmagnetic layer 4.

When the irradiated portion is further cooled down, the fourth magneticlayer 6 shows a magnetization. At this time, the magnetization of thefourth magnetic layer 6 is oriented in a downward direction by therecording magnetic field Hw. Since the fourth magnetic layer 6 israre-earth metal rich, the sublattice magnetization α of the transitionmetal is oriented in an upward direction (S6).

Further, when the irradiated portion is cooled down to a temperaturearound room temperature, the third magnetic layer 5 shows amagnetization. Therefore, exchange forces acting on the interfacesbetween the second magnetic layer 4 and the third magnetic layer 5 andbetween the third magnetic layer 5 and the fourth magnetic layer 6 aregenerated. The exchange forces orient the sublattice magnetization α ofthe transition metal of the second magnetic layer 4 in an upwarddirection that is the same direction as the direction of the sublatticemagnetization of the transition metal of the fourth magnetic layer 6.However, the direction of the magnetization of the first magnetic layer3 is not reversed by the magnetization of the second magnetic layer 4because the first magnetic layer 3 has a great coercive force at roomtemperature. Thus, the state "1" (downward magnetization) is recorded onthe first magnetic layer 3 (S7).

On the other hand, in the low process, laser light of low power (P1) isirradiated. As a result, the temperature of the irradiated portion isincreased to near Tc1. At this time, since the coercive force of thesecond magnetic layer 4 is stronger than the recording magnetic fieldHw, the direction of the magnetization of the second magnetic layer 4 isnot reversed by the recording magnetic field Hw. Thus, the orientationof the sublattice magnetization α of the transition metal of the secondmagnetic layer 4 is kept upward (S3).

When the portion irradiated with the light beam is cooled down by arotation of the magneto-optical disk, the first magnetic layer 3 shows amagnetization. At this time, the sublattice magnetization of thetransition metal of the first magnetic layer 3 is oriented in an upwarddirection that is the direction of the sublattice magnetization of thetransition metal of the second magnetic layer 4 by an exchange forceacting on the interface.

When the irradiated portion is further cooled down, the fourth magneticlayer 6 shows a magnetization. At this time, the magnetization of thefourth magnetic layer 6 is oriented in a downward direction by therecording magnetic field Hw. Since the fourth magnetic layer 6 israre-earth metal rich, the sublattice magnetization a of the transitionmetal is oriented in an upward direction (S2).

Further, when the irradiated portion is cooled down to near roomtemperature, the third magnetic layer 5 shows a magnetization. As aresult, exchange forces acting on the interfaces between the secondmagnetic layer 4 and the third magnetic layer 5 and between the thirdmagnetic layer 5 and the fourth magnetic layer 6 are generated. Theexchange forces orient the sublattice magnetization α of the transitionmetal of the second magnetic layer 4 in an upward direction that is thedirection of the sublattice magnetization of the transition metal of thefourth magnetic layer 6. However, the magnetization direction of thefirst magnetic layer 3 is not reversed by the magnetization of thesecond magnetic layer 4 since the first magnetic layer 3 has a highcoercive force at room temperature. Thus, a state "0" (upwardmagnetization) is recorded on the first magnetic layer 3 (S1).

As described above, the first magnetic layer 3 moves into a state "1" (adownward magnetization) in the case of the high process, and moves intoa state "0" (an upward magnetization) in the case of the low process,thereby achieving light-intensity modulation overwriting.

When reproducing information, laser light of reproduction power (Pr) isirradiated, and a rotatory polarization of the reflected light isdetected to perform reproduction. However, since the temperature of theirradiated portion is much lower than that in the low process, there isno possibility that the information is erased by the laser light of Pr.

In this embodiment, the second magnetic layer 4 has such acharacteristic that it is rare-earth metal rich at room temperature andhas its compensation point between room temperature and Tc2, and istransition metal rich at temperatures between Tc1 and Tc2. The fourthmagnetic layer 6 is rare-earth metal rich at room temperature and doesnot have its compensation point between room temperature and Tc2.However, the combination of the second magnetic layer 4 and the fourthmagnetic layer 6 is not necessarily limited to the one mentioned aboveif the kind of the sublattice magnetization of the second magnetic layer4 oriented in the direction of the recording magnetic field Hw by thehigh process and the kind of the sublattice magnetization of the fourthmagnetic layer 6 oriented in the direction of the recording magneticfield Hw at temperatures between Tc3 and Tc4 vary. For example, acombination of the second magnetic layer 4 showing perpendicularmagnetization and a rare-earth metal rich characteristic between roomtemperature and the Curie point Tc2 and the fourth magnetic layer 6showing perpendicular magnetization and a transition metal richcharacteristic between room temperature and the Curie point Tc4 may beadopted.

Since this combination is opposite to the above-mentioned combination,the open arrows in FIG. 3 which indicate the direction of the sublatticemagnetization of the transition metal in the above explanation need tobe considered as indicating the sublattice magnetization of therare-earth metal in this case. Since the first magnetic layer 3 istransition metal rich, in FIG. 3, the magnetization in S1 is a downwardmagnetization, and the magnetization in S7 is an upward magnetization.Thus, light-intensity modulation overwriting is also achieved in thiscase.

The third magnetic layer 5 smoothly copies the magnetization of thefourth magnetic layer 6 to the second magnetic layer 4 by adding theexchange force of the third magnetic layer 5 to the exchange force ofthe fourth magnetic layer 6. The magnetization of the third magneticlayer 5 is extinguished during the copying process of magnetization fromthe second magnetic layer 4 to the first magnetic layer 3. Therefore,the third magnetic layer 5 performs a function of preventing copying ofmagnetization from the fourth magnetic layer 6 to the second magneticlayer 4 during the process of copying magnetization from the secondmagnetic layer 4 to the first magnetic layer 3. As a result, since therequirements for magnetic properties such as the coercive forces andexchange forces of the first, second and fourth magnetic layers areeased, the materials for these magnetic layers can be selected from awider range.

Samples of the magneto-optical disk of this embodiment will be describedbelow.

Each of samples #1 and #2 uses disk-shaped glass with an outer diameterof 86 mm, an inner diameter of 15 mm and a thickness of 1.2 mm as thelight transmitting substrate 1. Guide tracks for guiding a light beamare directly produced in the form of grooves and lands on one surface ofthe substrate 1 by reactive ion etching. The guide tracks were directlyetched on the glass by reactive ion etching so as to achieve a trackpitch of 1.6 μm, a groove width of 0.8 μm, and a land width of 0.8 μm.

On the surface of the substrate 1 whereupon the guide tracks wereformed, a 80-nm-thick AlN film as the transparent dielectric layer 2 wasformed by reactive sputtering, a 40-nm-thick DyFeCo film as the firstmagnetic layer 3 was formed by simultaneously sputtering Dy, Fe and Cotargets, a 60-nm-thick GdFeCo film as the second magnetic layer 4 wasformed by simultaneously sputtering Gd, Fe and Co targets, a 20-nm-thickDyFe film as the third magnetic layer 5 was formed by simultaneouslysputtering Dy and Fe targets, a 40-nm-thick DyFeCo film as the fourthmagnetic layer 6 was formed by simultaneously sputtering Dy, Fe and Cotargets, and a 20-nm-thick AlN film as the protective layer 7 waslayered.

The first magnetic layer 3 to fourth magnetic layer 6 were formed underthe sputtering conditions of ultimate vacuum of not higher than 2.0×10⁻⁴Pa, Ar gas pressure of 6.5×10⁻¹ Pa, and discharge electric power of 300W. The transparent dielectric layer 2 and the protective layer 7 wereformed under the sputtering conditions of ultimate vacuum of not higherthan 2.0×10⁻⁴ Pa, N₂ gas pressure of 3.0×10⁻¹ Pa, and discharge electricpower of 800 W.

Moreover, an overcoat film is formed by placing an acrylate-seriesultraviolet curing resin over the protective layer 7 and curing theresin with ultraviolet irradiation.

The first magnetic layer 3 of #1 has a composition of Dy₀.20 (Fe₀.85CO₀.15)₀.08, is transition metal rich, has Curie point Tc1 of 180° C.,and coercive force Hc1 of 1200 kA/m at room temperature.

The second magnetic layer 4 has a composition of (Gd₀.60 Dy₀.40)₀.28(Fe₀.70 Co₀.30)₀.72, is rare-earth metal rich, has Curie point Tc2 of270° C., compensation point T_(comp3) of 200° C., and coercive force Hc2of 160 kA/m at room temperature.

The third magnetic layer 5 has a composition of Dy₀.18 Fe₀.82, istransition metal rich, has Curie point Tc3 of 70° C., and coercive forceHc3 of 200 kA/m at room temperature.

The fourth magnetic layer 6 has a composition of Dy₀.22 (Fe₀.90Co₀.10)₀.78, is rare-earth metal rich, has Curie point Tc4 of 150° C.,and coercive force Hc4 of 240 kA/m at room temperature.

The first magnetic layer 3 of #2 has a composition of Tb₀.20 (Fe₀.92Co₀.08)₀.80, is transition metal rich, has Curie point Tc1 of 180° C.,and coercive force Hc1 of 1200 kA/m at room temperature.

The second magnetic layer 4 has a composition of Tb₀.25 (Fe₀.80Co₀.20)₀.75, is rare-earth metal rich, has Curie point Tc2 of 270° C.,no compensation point, and coercive force Hc2 of 160 kA/m at roomtemperature.

The third magnetic layer 5 has a composition of Dy₀.18 Fe₀.82, istransition metal rich, has Curie point Tc3 of 70° C., and coercive forceHc3 of 200 kA/m at room temperature.

The fourth magnetic layer 6 has a composition of Dy₀.18 (Fe₀.90Co₀.10)₀.82, is rare-earth metal rich, has Curie point Tc4 of 150° C.,and coercive force Hc4 of 240 kA/m at room temperature.

Recording was performed on magneto-optical disks of samples #1 and #2under the conditions of Hw of 40 kA/m, Ph of 10 mW, Pl of 6 mW, Pr of 1mW, and a recording bit length of 0.64 μm. As a result, light-intensitymodulation overwriting was performed by completely erasing previousinformation, and a good signal to noise ratio (C/N) of 45 dB wasachieved.

Embodiment 2!

The following description will discuss another embodiment of the presentinvention with reference to FIGS. 4 to 6. The members having the samefunction as in the above-mentioned embodiment will be designated by thesame code and their description will be omitted.

The difference between the magneto-optical disk according to Embodiment1 and a magneto-optical disk (magneto-optical recording medium) of thisembodiment is that a fifth magnetic layer 8 is provided between thetransparent dielectric layer 2 and the first magnetic layer 3 as shownin FIG. 4 in this embodiment.

As illustrated in FIG. 5, the fifth magnetic layer 8 has a Curie point,Tc5, higher than the Curie point Tc1 of the first magnetic layer 3, andshows perpendicular magnetic anisotropy from room temperature to Tc5.

Referring now to FIG. 6, the following description will discuss therecording process of this embodiment. FIG. 6 shows the magnetizationstates of the fifth magnetic layer 8, first magnetic layer 3, secondmagnetic layer 4, third magnetic layer 5, and fourth magnetic layer 6.The horizontal axis in FIG. 6 indicates temperature. Since each layer isformed by an alloy of rare-earth metal and transition metal, totalmagnetization, and sublattice magnetization of rare-earthmetal/transition metal are present. The arrows shown in FIG. 6 indicatethe sublattice magnetization α of the transition metal of each layer.

The magnetization states of the first magnetic layer 3, second magneticlayer 4, third magnetic layer 5 and fourth magnetic layer 6 are the sameas those in the process of recording information on the magneto-opticaldisk of Embodiment 1 shown in FIG. 1, and therefore the explanationthereof will be omitted. Moreover, since the recording process in thisembodiment is substantially the same as that on the magneto-optical diskof Embodiment 1 shown in FIG. 3, the same explanation will not berepeated.

The magnetization state of the fifth magnetic layer 8 accords with themagnetization of the first magnetic layer 3 at temperatures not higherthan the Curie point Tc1 of the first magnetic layer 3.

In this embodiment, in a state S13 equivalent to the state S3 shown inFIG. 3, since the temperatures of the magnetic layers are not lower thanthe Curie point Tc1 of the first magnetic layer 3, the magnetization ofthe first magnetic layer 3 is extinguished. However, since the fifthmagnetic layer 8 has the Curie point T5 higher than the curie point Tc1of the first magnetic layer 3 as described above, the fifth magneticlayer 8 has magnetization at this temperature. Further, the fifthmagnetic layer 8 still has magnetization in a state S14 where thetemperature is higher than Tc2.

When each sublattice magnetization was copied from the second magneticlayer 4 to the first magnetic layer 3 in a state S16 in the course oflowering the temperature in the high process or a state S12 in thecourse of lowering the temperature in the low process, the sublatticemagnetization is also copied from the first magnetic layer 3 to thefifth magnetic layer 8.

In the reproduction process, since the temperatures of the respectivemagnetic layers are not higher than Tc1, the same information as theinformation recorded on the first magnetic layer 3 is reproduced throughthe fifth magnetic layer 8.

A sample of such a magneto-optical disk will be described below.

A sample #3 of the magneto-optical disk includes a 30-nm-thick fifthmagnetic layer 8 between the transparent dielectric layer 2 and thefirst magnetic layer 3, and was fabricated in the same method as thefabrication method of sample #1.

The fifth magnetic layer 8 of sample #3 has a composition of Gd₀.27(Fe₀.70 Co₀.30)₀.73, is rare-earth metal rich, has Curie point Tc5higher than 300° C., and a compensation point up to 200° C.

Recording was performed on the magneto-optical disks of sample #3 underthe same conditions as in Embodiment 1. As a result, light-intensitymodulation overwriting was performed by completely erasing previousinformation, and a good signal to noise ratio (C/N) of 46.5 dB wasachieved. Considering that the C/N ratio of sample #1 is 45 dB, it canbe said that the signal quality was improved compared to sample #1. Itis considered that the signal quality was improved because of anincrease in the Kerr rotation angle achieved by setting Tc5>Tc1.

Embodiment 3!

The following description will discuss another embodiment of the presentinvention with reference to FIGS. 7 to 9. The members having the samefunction as in the above-mentioned embodiment will be designated by thesame code and their description will be omitted.

The difference between the magneto-optical disks of the above-mentionedembodiments and a magneto-optical disk (magneto-optical recordingmedium) of this embodiment is that the fifth magnetic layer 8 isprovided between the transparent dielectric layer 2 and the firstmagnetic layer 3 as shown in FIG. 7 in this embodiment.

As illustrated in FIG. 8, the fifth magnetic layer 8 has the Curie pointTc5 higher than the Curie point Tc1 of the first magnetic layer 3,coercive force Hc5 of substantially zero at room temperature, and showsin-plane magnetic anisotropy at room temperature and perpendicularmagnetic anisotropy at temperatures not lower than a predeterminedtemperature (Tf).

Referring now to FIG. 9, the following description will discuss therecording process of this embodiment. FIG. 9 shows the magnetizationstates of the fifth magnetic layer 8, first magnetic layer 3, secondmagnetic layer 4, third magnetic layer 5, and fourth magnetic layer 6.The horizontal axis in FIG. 9 indicates temperature. Since each layer isformed by an alloy of rare-earth metal and transition metal, totalmagnetization and sublattice magnetization of rare-earthmetal/transition metal are present. The arrows shown in FIG. 9 indicatethe sublattice magnetization α of the transition metal of each layer.

The magnetization states of the first magnetic layer 3, second magneticlayer 4, third magnetic layer 5 and fourth magnetic layer 6 are the sameas those in the process of recording information on the magneto-opticaldisk of Embodiment 1 shown in FIG. 3, and therefore the explanationthereof will be omitted. Since the recording process in this embodimentis substantially the same as that on the magneto-optical disk ofEmbodiment 2 shown in FIG. 6, the same explanation will not be repeated.

The magnetization state of the fifth magnetic layer 8 shows in-planemagnetic anisotropy at room temperature and perpendicular magneticanisotropy at temperatures not lower than Tf. More specifically, instates S21 and S27 equivalent to the states S11 and S17 shown in FIG. 6,since the temperature is room temperature, the fifth magnetic layer 8shows in-plane magnetization. In the states other than S21 and S27 inFIG. 6, the fifth magnetic layer 8 shows perpendicular magnetization.The magnetization state of the fifth magnetic layer 8 accords with themagnetization state of the first magnetic layer 3 at temperatures of notlower than Tf and not higher than the Curie point Tc1 of the firstmagnetic layer 3.

In the reproduction process, since the temperatures of the respectivemagnetic layers are between Tf and Tc1, the same information as theinformation recorded on the first magnetic layer 3 is reproduced throughthe fifth magnetic layer 8.

A sample of such a magneto-optical disk will be described below.

A sample #4 of the magneto-optical disk includes a 30-nm-thick fifthmagnetic layer 8 between the transparent dielectric layer 2 and thefirst magnetic layer 3 of sample #1, and was fabricated in the samemethod as the fabrication method of sample #1.

The fifth magnetic layer 8 of sample #4 has a composition of Gd₀.29(Fe₀.80 Co₀.20)₀.71, is rare-earth metal rich, has Curie point Tc5 of300° C., no compensation point, and shows perpendicular magneticanisotropy at about 120° C.

Recording was performed on the magneto-optical disk of sample #4 underthe same conditions as in Embodiment 1. As a result, light-intensitymodulation overwriting was performed by completely erasing previousinformation, and a good signal to noise ratio (C/N) of 46 dB wasachieved. Considering that the C/N ratio of sample #1 is 45 dB, it canbe said that the signal quality was improved compared to sample #1. LikeEmbodiment 2, the signal quality was improved because of an increase inthe Kerr rotation angle achieved by setting Tc5>Tc1.

In addition, when the recording bit length became shorter, the C/N ratiowas abruptly lowered in sample #1, but it was not lowered much in sample#4. The reason for this would be that since the fifth magnetic layer 8shows in-plane magnetic anisotropy at room temperature and showsperpendicular magnetization with the irradiation of laser light ofreproducing laser power, even if a recording bit is short, reproductioncan be performed without being affected by adjacent recording bits.

Embodiment 4!

The following description will discuss another embodiment of the presentinvention with reference to FIGS. 10 to 12. The members having the samefunction as in the above-mentioned embodiment will be designated by thesame code and their description will be omitted.

The difference between the magneto-optical disks of the above-mentionedembodiments and a magneto-optical disk (magneto-optical recordingmedium) of this embodiment is that a sixth magnetic layer 9 is providedbetween the first magnetic layer 3 and the second magnetic layer 4 asshown in FIG. 10 in this embodiment.

As illustrated in FIG. 11, the sixth magnetic layer 9 has a coerciveforce Hc6 of substantially zero at room temperature, and shows very weakperpendicular or in-plane magnetic anisotropy at room temperature andperpendicular magnetic anisotropy at temperatures not lower than apredetermined temperature (Ts).

Referring now to FIG. 12, the following description will discuss therecording process of this embodiment. FIG. 12 shows the magnetizationstates of the first magnetic layer 3, sixth magnetic layer 9, secondmagnetic layer 4, third magnetic layer 5, and fourth magnetic layer 6.The horizontal axis in FIG. 12 indicates temperature. Since each layeris formed by an alloy of rare-earth metal and transition metal, totalmagnetization and sublattice magnetization of rare-earthmetal/transition metal are present. The arrows shown in FIG. 12 indicatethe sublattice magnetization α of the transition metal of each layer.

The magnetization states of the first magnetic layer 3, second magneticlayer 4, third magnetic layer 5 and fourth magnetic layer 6 are the sameas those in the process of recording information on the magneto-opticaldisk of Embodiment 1 shown in FIG. 3, and therefore the explanationthereof will be omitted. Moreover, since the recording process in thisembodiment is substantially the same as that on the magneto-optical diskof Embodiment 1 shown in FIG. 3, the same explanation will not berepeated.

The magnetization state of the sixth magnetic layer 9 shows very weakperpendicular magnetic anisotropy or in-plane magnetic anisotropy atroom temperature and strong perpendicular magnetic anisotropy attemperatures not lower than Ts. Therefore, copying of magnetization fromthe second magnetic layer 4 to the first magnetic layer 3 is not readilyperformed at room temperature, and copying of magnetization from thesecond magnetic layer 4 to the first magnetic layer 3 is carried out attemperatures not lower than Ts. Therefore, as described below, therespective magnetization states become more stable, and light-intensitymodulation overwriting is performed more smoothly compared to themagneto-optical disk of Embodiment 1.

In this embodiment, when copying the magnetization (each sublatticemagnetization) from the second magnetic layer 4 to the first magneticlayer 3 in a state S36 in the course of lowering the temperature in thehigh process or a state S32 in the course of lowering the temperature inthe low process, the sixth magnetic layer 9 functions as an intermediaryand the direction of the sublattice magnetization is copied from thesecond magnetic layer 4 to the sixth magnetic layer 9. Then, thedirection of the sublattice magnetization is copied from the sixthmagnetic layer 9 to the first magnetic layer 3.

In the following state S37 of the high process and S38 in the course ofincreasing the temperature for next recording, the sublatticemagnetization of the transition metal of the first magnetic layer 3 isoriented in a downward direction and the sublattice magnetization of thetransition metal of the second magnetic layer 4 is oriented in an upwarddirection. Accordingly, the sublattice magnetization of the transitionmetal of the sixth magnetic layer 9 shows in-plane magnetization as amore stable direction.

On the other hand, in the following state S31 of the low process or S32in which the temperature is increased from S31 where "0" is recorded,since the sublattice magnetization of the transition metal of the firstmagnetic layer 3 and the sublattice magnetization of the transitionmetal of the second magnetic layer 4 are oriented in an upwarddirection, the sublattice magnetization of the transition metal of thesixth magnetic layer 9 shows perpendicular magnetization in the samedirection as a more stable direction.

In a state S34 equivalent to the state S4 shown in FIG. 3, since thetemperatures of the respective magnetic layers are not lower than theCurie point Tc2 of the second magnetic layer 4, the magnetization ofeach of the first to fourth magnetic layers is extinguished. However,since the sixth magnetic layer 9 has the Curie point Tc6 higher than theCurie point Tc2, the sixth magnetic layer 9 has a magnetization at thistemperature.

A sample of such a magneto-optical disk will be described below.

A sample #5 of the magneto-optical disk includes a 40-nm-thick sixthmagnetic layer 9 between the first magnetic layer 3 and the secondmagnetic layer 4 of sample #1, and was fabricated in the same method asthe fabrication method of sample #1.

The sixth magnetic layer 9 of sample #5 has a composition of Gd₀.27(Fe₀.70 Co₀.30)₀.73, is rare-earth metal rich, has Curie point Tc6higher than 300° C., and a compensation point up to 200° C.

Recording was performed on the magneto-optical disk of sample #5 underthe conditions of Hw of 32 kA/m, Ph of 9 mW, Pl of 1 mW, and a recordingbit length of 0.64 μm. As a result, light-intensity modulationoverwriting was performed by completely erasing previous information,and a good signal to noise ratio (C/N) of 45 dB was achieved.Considering that the recording conditions of sample #1 are Hw of 40kA/m, Ph of 10 mW and Pl of 6 mW, the recording sensitivity was improvedcompared to sample #1. It is considered that such an improvement wasachieved because light-intensity modulation overwriting was smoothlyperformed by inserting the sixth magnetic layer 9 between the firstmagnetic layer 3 and the second magnetic layer 4.

Embodiment 5!

The following description will discuss another embodiment of the presentinvention with reference to FIGS. 13 to 15. The members having the samefunction as in the above-mentioned embodiment will be designated by thesame code and their description will be omitted.

The difference between the magneto-optical disks of the above-mentionedembodiments and a magneto-optical disk (magneto-optical recordingmedium) of this embodiment is that the fifth magnetic layer 8 isprovided between the transparent dielectric layer 2 and the firstmagnetic layer 3 and the sixth magnetic layer 9 is provided between thefirst magnetic layer 3 and the second magnetic layer 4 as shown in FIG.13 in Embodiment 5.

As illustrated in FIG. 14, the fifth magnetic layer 8 has Curie pointTc5 higher than the Curie point Tc1 of the first magnetic layer 3 andshows perpendicular magnetic anisotropy from room temperature to Tc5.

The sixth magnetic layer 9 has a coercive force Hc6 of substantiallyzero at room temperature, shows very weak perpendicular magneticanisotropy or in-plane magnetic anisotropy at room temperature, andperpendicular magnetic anisotropy at temperatures not lower than thepredetermined temperature Ts.

FIG. 15 shows the magnetization states of the fifth magnetic layer 8,first magnetic layer 3, sixth magnetic layer 9, second magnetic layer 4,third magnetic layer 5, and fourth magnetic layer 6. The horizontal axisin FIG. 15 indicates temperature. Since each layer is formed by an alloyof rare-earth metal and transition metal, total magnetization andsublattice magnetization of rare-earth metal/transition metal arepresent. The arrows shown in FIG. 15 indicate the sublatticemagnetization α of the transition metal of each layer.

The magnetization states of the first magnetic layer 3, second magneticlayer 4, third magnetic layer 5 and fourth magnetic layer 6 are the sameas those in the process of recording information on the magneto-opticaldisk of Embodiment 1 shown in FIG. 3, and therefore the explanationthereof will be omitted. Similarly, since the magnetization states ofthe fifth magnetic layer 8 and sixth magnetic layer 9 are the same asthose in Embodiments 2 and 4, respectively, the explanation thereof willbe omitted.

A sample of such a magneto-optical disk will be described below.

A sample #6 of the magneto-optical disk includes a 30-nm-thick fifthmagnetic layer 8 between the transparent dielectric layer 2 and thefirst magnetic layer 3 of sample #1, and a 40-nm-thick sixth magneticlayer 9 between the first magnetic layer 3 and the second magnetic layer4. The sample #6 was fabricated in the same method as the fabricationmethod of sample #1.

Recording was performed on the magneto-optical disk of sample #6 underthe conditions of Hw of 32 kA/m, Ph of 9 mW, Pl of 4 mW, Pr of 1 mW, anda recording bit length of 0.64 μm. As a result, light-intensitymodulation overwriting was performed by completely erasing previousinformation, and a good signal to noise ratio (C/N) of 46.5 dB wasachieved. Considering that the recording conditions of sample #1 are Hwof 40 kA/m, Ph of 10 mW and Pl of 6 mW, the recording sensitivity wasimproved compared to sample #1. It is considered that such animprovement was achieved because light-intensity modulation overwritingwas smoothly performed by inserting the sixth magnetic layer 9 betweenthe first magnetic layer 3 and the second magnetic layer 4. In addition,considering that C/N ratio of sample #1 is 45 dB, the signal quality wasimproved compared to sample #1. It is considered that the signal qualitywas improved because of an increase in the Kerr rotation angle achievedby setting Tc5>Tc1.

In Embodiments 1 to 5 above, glass was used as the substrate 1 ofsamples #1 to #6. Alternatively, it is possible to use chemicallyreinforced glass, so-called 2P layered glass produced by forming aultraviolet curing resin layer on the substrate 1, polycarbonate (PC),polymethyl methacrylate (PMMA), amorphous polyolefin (APO), polystyrene(PS), polyvinyl chloride (PVC), and epoxy as the substrate 1.

The thickness of the AlN film of the transparent dielectric layer 2 isnot restricted to 80 nm. The film thickness of the transparentdielectric layer 2 is determined considering the enhancement of aso-called Kerr effect, i.e., an increase in the polar Kerr rotationangle from the first magnetic layer 3 or the fifth magnetic layer 8 bythe interference effect of light when reproducing the magneto-opticaldisk. In order to maximize the C/N ratio during reproduction, it isnecessary to increase the polar Kerr rotation angle. Therefore, the filmthickness of the transparent dielectric layer 2 is set so as to increasethe polar Kerr rotation angle.

The transparent dielectric layer 2 not only enhances the Kerr effect,but also prevents the oxidation of the first magnetic layer 3 to fourthmagnetic layer 6, fifth magnetic layer 8 or sixth magnetic layer 9 ofalloys of rare-earth metal and transition metal together with theprotective layer 7.

Moreover, AlN permits reactive DC (direct current power source)sputtering using an Al target by introducing N₂ gas or a mixed gas of Arand N₂, and has the advantage of faster film forming speed compared toRF (high frequency) sputtering.

Preferred examples of the material of the transparent dielectric layer 2other than AlN include SiN, AlSiN, AlTaN, SiAlON, TiN, TiON, BN, ZnS,TiO₂, BaTiO₃, and SrTiO₃. Among these materials, SiN, AlSiN, AlTaN, TiN,BN and ZnS do not contain oxygen, thereby providing a magneto-opticaldisk of excellent moisture-proof characteristics.

The material and composition of the first magnetic layer 3 to fourthmagnetic layer 6, fifth magnetic layer 8 or sixth magnetic layer 9 ofalloys of rare-earth metal and transition metal are not restricted tothose mentioned above. Similar effects can be produced by using an alloymade of at least one kind of rare-earth metal selected from the groupconsisting of Gd, Tb, Dy, Ho and Nd and at least one kind of transitionmetal selected from the group consisting of Fe and Co as the materialfor the first magnetic layer 3 to fourth magnetic layer 6, fifthmagnetic layer 8 or sixth magnetic layer 9.

The resistance of the first magnetic layer 3 to fourth magnetic layer 6,fifth magnetic layer 8 or sixth magnetic layer 9 to environment can beimproved by adding at least one kind of element selected from the groupconsisting of Cr, V, Nb, Mn, Be, Ni, Ti, Pt, Rh and Cu. It is thuspossible to reduce the deterioration of the characteristics due to theoxidation caused by penetrated moisture or oxygen, and provide amagneto-optical disk that is reliable over a long period of time.

The film thickness of the first magnetic layer 3 to fourth magneticlayer 6, fifth magnetic layer 8 or sixth magnetic layer 9 of alloys ofrare-earth metal and transition metal is not restricted to thosementioned above, and is determined depending on the material andcomposition thereof.

Although the thickness of the AlN film of the protective layer 7 was setto 80 nm in the embodiments, the film thickness is not necessarilyrestricted to this value. A preferred range of the film thickness of theprotective layer 7 is between 1 nm and 200 nm.

The thermal conductivity of the protective layer 7 as well as thetransparent dielectric layer 2 affects the recording sensitivity of themagneto-optical disk. The recording sensitivity indicates the degree oflaser power necessary for recording or erasing. The light incident uponthe magneto-optical disk mostly passes through the transparentdielectric layer 2, is absorbed by the first magnetic layer 3 to fourthmagnetic layer 6, fifth magnetic layer 8 or sixth magnetic layer 9, andconverted to heat. At this time, the heat of the first magnetic layer 3to fourth magnetic layer 6, fifth magnetic layer 8 or sixth magneticlayer 9 is moved to the transparent dielectric layer 2 and theprotective layer 7 by the conduction of heat. Accordingly, the thermalconductivity and heat capacity (specific heat) of the transparentdielectric layer 2 and the protective layer 7 affect the recordingsensitivity.

This means that the recording sensitivity of the magneto-optical diskcan be controlled to some extent by the film thickness of the protectivelayer 7. For example, a reduction of the film thickness of theprotective layer 7 is necessary for the purpose of increasing therecording sensitivity (i.e., perform recording and erasing with lowlaser power). In general, in order to extend the life of the laser, ahigh recording sensitivity is advantageous and the protective layer 7with a small film thickness is preferred.

Since AlN is suitable in this sense and has high resistance to moisture,the use of AlN as the protective layer 7 enables a reduction in the filmthickness and a magneto-optical disk with high recording sensitivity. Inthis embodiment, when the protective layer 7 is formed by AlN that isused for the transparent dielectric layer 2, it is possible to provide amagneto-optical disk with high resistance to moisture. Moreover, sincethe protective layer 7 and the transparent dielectric layer 2 are formedby the same material, the productivity is improved.

Considering the above purpose and effects, preferred materials for theprotective layer 7 are the above-mentioned materials used for thetransparent dielectric layer 2, namely, SiN, AlSiN, AlTaN, SiAlON, TiN,TiON, BN, ZnS, TiO₂, BaTiO₃, and SrTiO₃. Among these materials, SiN,AlSiN, AlTaN, TiN, BN and ZnS do not contain oxygen, thereby providing amagneto-optical disk of excellent moisture-proof characteristics.

Samples #1 to #6 of magneto-optical disk are generally called"single-sided disks". If the thin film section including the transparentdielectric layer 2, first magnetic layer 3 to fourth magnetic layer 6,fifth magnetic layer 8 or sixth magnetic layer 9, and protective layer 7is referred to as a recording medium layer, a single-sidedmagneto-optical disk is constructed by the substrate 1, recording mediumlayer and overcoat layer.

On the other hand, a magneto-optical disk formed by positioning twopieces of substrates 1 whereon the recording medium layers are formed,respectively, to face each other and fastening them with an adhesivelayer is called a "double-sided disk". A polyurethane series adhesiveagent is particularly preferred as a material for the adhesive layer.This adhesive agent has a combination of three types of hardeningfunctions, i.e., ultraviolet-hardening, thermosetting and anaerobicproperties. Such a combination gives such an advantage that a portionshaded by the recording medium where ultraviolet rays do not pass ishardened by thermosetting and anaerobic hardening functions. It is thuspossible to provide a magneto-optical disk which has very highresistance to moisture and excellent stability over a long period oftime.

The thickness of the elements constituting the single-sided disk is ahalf of that of the double-sided disk, the single-sided disk isadvantageous to, for example, a recording and reproducing device whoseobject is to reduce the size. On the other hand, the double-sided diskenables reproduction from both sides, and is therefore advantageous to,for example, a recording and reproducing device whose object is toincrease the capacity.

In the above-mentioned embodiments, the magneto-optical disks areexplained as an example of the magneto-optical recording medium.However, the present invention is also applicable to magneto-opticaltapes and magneto-optical cards.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A magneto-optical recording medium comprising afirst magnetic layer, a second magnetic layer and a fourth magneticlayer having curie points Tc1, Tc2 and Tc4, respectively, and showingperpendicular magnetization from room temperature to the Curie pointsTc1, Tc2, and Tc4, said first, second and fourth magnetic layers beingarranged in this order,a direction of magnetization of said secondmagnetic layer being copied to said first magnetic layer by an exchangeforce at temperatures between room temperature and Tc1, a direction ofmagnetization of said fourth magnetic layer being copied to said secondmagnetic layer by an exchange force but magnetization of said secondmagnetic layer being not copied to said first magnetic layer in apredetermined temperature range R between room temperature and Tc4, saidroom temperature, Tc1, Tc2 and Tc4 being related by

    room temperature<Tc4<Tc1<Tc2,

wherein a fifth magnetic layer whose Curie point Tc5 is higher than theCurie point Tc1 of said first magnetic layer is provided on a side ofsaid first magnetic layer, opposite to a side whereupon said secondmagnetic layer is formed.
 2. The magneto-optical recording mediumaccording to claim 1,wherein said fifth magnetic layer showsperpendicular magnetization from room temperature to Tc5.
 3. Themagneto-optical recording medium according to claim 1,wherein said fifthmagnetic layer shows in-plane magnetization at room temperature, andperpendicular magnetization at a predetermined temperature Tf which ishigher than room temperature but lower than Tc1.
 4. The magneto-opticalrecording medium according to claim 1,wherein said fifth magnetic layeris formed by an alloy of rare-earth metal and transition metal asferrimagnetic material.
 5. A magneto-optical recording medium comprisinga first magnetic layer, a second magnetic layer and a fourth magneticlayer having curie points Tc1, Tc2 and Tc4, respectively, and showingperpendicular magnetization from room temperature to the Curie pointsTc1, Tc2, and Tc4, said first, second and fourth magnetic layers beingarranged in this order,a direction of magnetization of said secondmagnetic layer being copied to said first magnetic layer by an exchangeforce at temperatures between room temperature and Tc1, a direction ofmagnetization of said fourth magnetic layer being copied to said secondmagnetic layer by an exchange force but magnetization of said secondmagnetic layer being not copied to said first magnetic layer in apredetermined temperature range R between room temperature and Tc4, saidroom temperature, Tc1, Tc2 and Tc4 being related by

    room temperature<Tc4<Tc1<Tc2,

wherein a sixth magnetic layer, showing in plane magnetization at roomtemperature and perpendicular magnetization at a temperature near Tc1,is provided between said first magnetic layer and said second magneticlayer.
 6. The magneto-optical recording medium according to claim5,wherein said sixth magnetic layer has a Curie point Tc6 higher thanTc2.
 7. The magneto-optical recording medium according to claim5,wherein said sixth magnetic layer is made of an alloy of rare-earthmetal and transition metal as ferrimagnetic material.
 8. Themagneto-optical recording medium according to claim 1,wherein saidsecond and fourth magnetic layers are formed by alloys of rare-earthmetal and transition metal as ferrimagnetic materials, if sublatticemagnetization of one of the transition metal and the rare-earth metal isindicated as α and the other as β, α is stronger than β in said secondmagnetic layer at temperatures between Tc1 and Tc2, and β is strongerthan α in said fourth magnetic layer at temperatures between roomtemperature and Tc4.
 9. The magneto-optical recording medium accordingto claim 1,wherein said first magnetic layer is made of an alloy ofrare-earth metal and transition metal as ferrimagnetic material, and thesublattice magnetization of the transition metal is stronger than thesublattice magnetization of the rare-earth metal at temperatures betweenroom temperature and Tc1.
 10. The magneto-optical recording mediumaccording to claim 1,wherein, in said second magnetic layer, thesublattice magnetization of the rare-earth metal is stronger than thesublattice magnetization of the transition metal at room temperature andthe sublattice magnetization of the transition metal is stronger thanthe sublattice magnetization of the rare-earth metal at temperaturesbetween Tc1 and Tc2, and in said fourth magnetic layer, the sublatticemagnetization of the rare-earth metal is stronger than the sublatticemagnetization of the transition metal at temperatures between roomtemperature and Tc4.
 11. The magneto-optical recording medium accordingto claim 10,wherein said second magnetic layer has a compensation pointbetween room temperature and Tc2.
 12. The magneto-optical recordingmedium according to claim 11,wherein said compensation point is nearTc1.
 13. The magneto-optical recording medium according to claim1,wherein, in said second magnetic layer, the sublattice magnetizationof the rare-earth metal is stronger than the sublattice magnetization ofthe transition metal at temperatures between room temperature and Tc2,and in said fourth magnetic layer, the sublattice magnetization of thetransition metal is stronger than the sublattice magnetization of therare-earth metal at temperatures between room temperature and Tc4. 14.The magneto-optical recording medium according to claim 1,wherein athird magnetic layer is provided between said second magnetic layer andsaid fourth magnetic layer, said third magnetic layer showingperpendicular magnetization from room temperature to its Curie pointTc3, Tc3 being related to room temperature and Tc4 by

    room temperature<Tc3<Tc4,

said temperature range R is a range between room temperature and Tc3,and the magnetization of said fourth magnetic layer is copied to saidthird magnetic layer by an exchange force and the magnetization of saidthird magnetic layer is copied to said second magnetic layer by anexchange force in said temperature range R.
 15. The magneto-opticalrecording medium according to claim 14,wherein, said third magneticlayer is made of an alloy of rare-earth metal and transition metal asferrimagnetic material, and the sublattice magnetization of thetransition metal is stronger than the sublattice magnetization of therare-earth metal at temperatures between room temperature and Tc3. 16.The magneto-optical recording medium according to claim 5,wherein saidsecond and fourth magnetic layers are formed by alloys of rare-earthmetal and transition metal as ferrimagnetic materials, if sublatticemagnetization of one of the transition metal and the rare-earth metal isindicated as α and the other as β, α is stronger than β in said secondmagnetic layer at temperatures between Tc1 and Tc2, and β is strongerthan α in said fourth magnetic layer at temperatures between roomtemperature and Tc4.
 17. The magneto-optical recording medium accordingto claim 5,wherein said first magnetic layer is made of an alloy ofrare-earth metal and transition metal as ferrimagnetic material, and thesublattice magnetization of the transition metal is stronger than thesublattice magnetization of the rare-earth metal at temperatures betweenroom temperature and Tc1.
 18. The magneto-optical recording mediumaccording to claim 5,wherein, in said second magnetic layer, thesublattice magnetization of the rare-earth metal is stronger than thesublattice magnetization of the transition metal at room temperature andthe sublattice magnetization of the transition metal is stronger thanthe sublattice magnetization of the rare-earth metal at temperaturesbetween Tc1 and Tc2, and in said fourth magnetic layer, the sublatticemagnetization of the rare-earth metal is stronger than the sublatticemagnetization of the transition metal at temperatures between roomtemperature and Tc4.
 19. The magneto-optical recording medium accordingto claim 18,wherein said second magnetic layer has a compensation pointbetween room temperature and Tc2.
 20. The magneto-optical recordingmedium according to claim 19,wherein said compensation point is nearTc1.
 21. The magneto-optical recording medium according to claim5,wherein, in said second magnetic layer, the sublattice magnetizationof the rare-earth metal is stronger than the sublattice magnetization ofthe transition metal at temperatures between room temperature and Tc2,and in said fourth magnetic layer, the sublattice magnetization of thetransition metal is stronger than the sublattice magnetization of therare-earth metal at temperatures between room temperature and Tc4. 22.The magneto-optical recording medium according to claim 5,wherein athird magnetic layer is provided between said second magnetic layer andsaid fourth magnetic layer, said third magnetic layer showingperpendicular magnetization from room temperature to its Curie pointTc3, Tc3 being related to room temperature and Tc4 by

    room temperature<Tc3<Tc4,

said temperature range R is a range between room temperature and Tc3,and the magnetization of said fourth magnetic layer is copied to saidthird magnetic layer by an exchange force and the magnetization of saidthird magnetic layer is copied to said second magnetic layer by anexchange force in said temperature range R.
 23. The magneto-opticalrecording medium according to claim 22,wherein, said third magneticlayer is made of an alloy of rare-earth metal and transition metal asferrimagnetic material, and the sublattice magnetization of thetransition metal is stronger than the sublattice magnetization of therare-earth metal at temperatures between room temperature and Tc3.